1. Field of the Invention
The present invention relates to the field of kaolin clay products, including delaminated and nondelaminated kaolin clay pigment products, which are particularly useful for use in paper products. Specifically, the present invention relates to kaolin clay products having high brightness and good opacity characteristics, and methods of making and using such products.
2. Background Art
Kaolin clay pigments are obtained from kaolin crudes. The crudes can contain kaolin particles, oversize particle size impurities (grit), and fine and coarse particle size impurities, such as fine ferruginous or titaniferous impurities, which impart undesirable color to the clay. The kaolin particles typically occur in the crude over a wide range of particle sizes and shapes. Typically a kaolin crude, such as a typical Georgia crude will contain, after removing the grit, particles ranging in size from submicron or colloidal to 20 microns or larger. Typically the kaolin morphology includes arrangement in plates, which plates can be further arrayed in stacks. Particle size, impurity content and morphology can vary with location of the deposit and within any given deposit itself.
Particle size of kaolins are typically determined by sedimentation using Stokes law to convert settling rates to particle size distribution and assuming a spherical shape of the particles. Measurements of particle size determined by this method therefore are termed as e.s.d., i.e. equivalent spherical diameter, as determined by Sedigraph. Typically, kaolin particles finer than about 2 microns are composed of individual platelets, and particles larger than about 2 microns are composed of stacks or booklets of finer particles.
Previous methods of producing kaolin clay pigments for paper end uses have included steps such as delamination, which disaggregates large kaolin particles, i.e., stacks, into smaller particles having a higher aspect ratio, primarily by separating clay crystals along basal cleavages. See, e.g., U.S. Pat. Nos. 3,171,718 (Gunnet et al.); U.S. Pat. No. 5,169,443 (Willis et al.), U.S. Pat. No. 5,411,587 (Willis et al.). Typically, however, the resulting product has a brightness of around 84-85. For end uses such as use as paper coating, a kaolin having a higher brightness, and also possessing other desirable characteristics such as good opacity, would be highly advantageous for making a superior paper product. For end uses such as use as paper filling a kaolin having a higher brightness, and also possessing other desirable characteristics such as good opacity and good tensile strength, would be highly advantageous for making a superior yet economical paper product.
In paper production, currently precipitated calcium carbonate (PCC) is used as a filler in paper to reduce the percentage of pulp that must be used as filler in the paper, pulp being expensive. Precipitated calcium carbonate has been used as a filler to reduce that cost. Typically, precipitated calcium carbonate has been used at percentages around 12% to 15%. When precipitated calcium carbonate content was increased to about 18%, a reduction in paper tensile strength resulted. Therefore, PCC as a filler has been limited to about 12-15% by its degrading influence on physical properties.
The present invention provides both delaminated and nondelaminated kaolin pigments having surprising brightness of 87 or greater and even 90 or greater, and methods of making such kaolin pigments. These pigments further have good opacity and strong tensile strength. Thus the present invention provides a highly desirable pigment useful in any application wherein high brightness, opaque pigments are needed, and particularly in uses such as in paper coating applications, and in paper filling applications wherein pigments having not only high brightness and good opacity but also good tensile strength are needed.